Security documents, including value documents such as banknotes and cheques, and identity documents such as passports and identity cards, are frequently subjected to fraud. In order to increase security and to make forgery more difficult, security documents are frequently provided with so-called security elements, applied onto or inserted into said security documents. The security elements can, for example, provide controlled responses to external stimuli, and/or provide certain visual effects, thereby allowing for the verification of the authenticity of the document into which they have been incorporated.
The configuration, design and composition of a security element is often intended to remain unaltered once it has been incorporated into the security document, for example, applied to or inserted into a substrate of the security document, such as a paper substrate of the security document. Generally, the security element is intended to maintain its properties until the end of the life cycle of the security document, although sometimes alterations may occur due to, for example, wear and tear caused by the use of the document.
Security elements can be in the form of, for example, security threads or strips, luminescent fibres, iridescent strips or planchettes, holographic tags, patches or strips, solid microparticles, reactive chemical agents or printed security inks. It is known to place this kind of elements inside the paper substrate of a document or within the pulp from which the substrate is produced (this is often the case with security threads, strips, fibres, microparticles or reactive chemical agents) or on the surface of the substrate (this is often the case with patches, holographic strips, reactive chemical agents or printed strips). It is known to place security elements at specific positions of a substrate and/or in register with other security features of a substrate (this is frequently the case with security threads and strips, holographic strips, and printed inks), but security elements can also be distributed randomly on or in the substrate (this is often the case with fibres, solid microparticles, and reactive chemical agents).
The incorporation of the security elements into the security document contributes to making forgery more difficult in at least two different ways, namely: due to the difficulty of manufacturing the security elements; due to the difficulty of incorporating the element into the substrate (especially if placed in register with other features). The level of difficulty can be further enhanced if several security elements are incorporated in the same security document, and especially of they are placed very close to each other.
The position in register or the precise positioning of each security element in a substrate makes it possible to incorporate a relatively large amount of security elements in the substrate. On the contrary, when there are substantial tolerances in the positioning of a security element in a substrate, it may be necessary to increase the space between security elements so as to reduce the risk of non-desired interaction or overlapping between security elements; this implies a restriction on the amount of security elements that can be incorporated into and/or onto the substrate of the security document.
A large number of prior art references teach different aspects related to the incorporation of security threads or ribbons into paper substrates.
For example, GB-1095286-A discloses the use of thin security ribbons which are incorporated into a security paper. Said ribbons contain, before their incorporation into the paper, different graphical designs in the form of characters or symbols which can also be visually detected, through the use of lenses or microscopes, once incorporated into the paper.
WO-2004/050991-A1 discloses a method for manufacturing a security paper in which a security thread is partially embedded in such a way that there are areas of the said security thread which remain exposed. The holographic or metallized graphical motifs can thus be seen with the naked eye, in the same way in which they could be seen on the thread before it was inserted into the paper substrate. In addition, it is described that this thread will be positioned at a specific distance from (that is, in register with) a watermark, in order to facilitate the control of the position.
One problem frequently involved with the use of security threads is that the substrate is deformed by the security thread, as the presence of the security thread in the substrate increases the thickness of the substrate locally, as the thickness of the security thread is added to the thickness of the paper as such. This is the reason for why, in bank notes, the position of the security thread often varies between different individual bank notes of the same kind: frequently, the position of the security thread in the bank note, in the cross direction (that is, the direction perpendicular to the machine direction, which corresponds to the axial direction of the security thread), can vary several mm if different banknotes are compared. This prevents the increased thickness of the individual banknotes at the location of the security thread to accumulate and produce an excessive bulging of a stack of banknotes comprising a large number of superposed banknotes of the same kind.
However, this implies a difficulty when a certain security feature of a security thread, such as an image, symbol, marking, or other characteristic of the security thread, or the security thread itself, is to be placed in register with a security feature which forms part of the substrate of the document, such as an image, symbol or other mark printed on a paper substrate. For example, if an image printed on a security thread is to be placed in register with an image printed on a substrate into which the thread is to be inserted, this may be difficult or impossible if the position of the security thread is not the same in all substrates, such as, for example, in the paper substrates of a plurality of bank notes.
Also, when substrates for security documents are produced by cutting from a sheet or bobbin of the substrate material already containing the security thread (such as a sheet or web of paper or other cellulose based material), tolerances in the cutting process may affect the position of the security thread within the individual substrate, for example, in relation to the edge of the substrate. That is, if comparing a plurality of cut substrates, the security thread may not always be in the same position in relation to an edge or side of the substrate, for example, in the case of bank notes, typically in relation to one of the shorter sides, which often are parallel with the security thread.
Sometimes, an intended variation in the position of the security thread (for example, as in the case of banknotes, for the purpose of preventing all of the security threads from being exactly superimposed on top of each other when stacking banknotes, so as to prevent the stack from bulging excessively) can add up with a variation due to tolerances in the processes of insertion of the security threads into the substrates and/or cutting of the substrates, thereby giving rise to a substantial variability of the position of the security thread in relation to a reference point of a substrate, such as an edge of the substrate.
That is, to prevent the locally increased thickness of the substrate to accumulate excessively when many substrates are placed on top of each other, and in order to avoid the risk for “non-valid” substrates due to the tolerances in the insertion of the threads and/or cutting of the substrates from a large sheet or band, it is known to vary the position of the security thread in a controlled way within a predetermined interval, so that if comparing a number of substrates, the thread will not always be in the same position within the document (for example, in relation to an edge or side of the substrate or security document): the thread will be placed in a position that can vary within a given range, for example, with +/− a few millimeters from a reference position. However, this varying position of the thread may raise doubts with regard to the validity or authenticity of the document, for example, when a layman examines two banknotes and observes that the security threads are not placed in the same position.
Also, as indicated above, a further problem involved with a lack of register between a security element and a substrate is that it restricts the possibility of adding further security elements, due to the risk of interference between different security elements when they are very near each other of when they overlap each other.
EP-1872965-A1 teaches a security thread, strip or band comprising a cellulose support which can act as a carrier for security elements such as pigments, synthetic elements and/or security fibers, and which can be inserted into a paper substrate, whereby the cellulose substrate of the security strip can be completely integrated in the paper pulp, although without disappearing as an independent element. The fact that both the substrate of the security strip as the paper substrate into which it is integrated, that is, the substrate of the security document, to a substantial extent are made up of cellulose fibres, facilitates the integration between the substrate of the security document (hereinafter also referred to as the “document substrate”) and the substrate of the security element (hereinafter also referred to as the “element substrate”). Due to this integration, such a cellulose strip does not contribute to an increase in the thickness of the document substrate in the same way as, for example, a metal or plastic strip. The strip can be provided with detectable symbols or other security features.
A known way of arranging a security feature of a security element in register with a substrate includes forcing, in a controlled manner, the variation of the visual appearance of the security element after it has been applied to the substrate.
For instance, US-2008/0191462-A1 discloses a security document with a paper substrate, having a coating on a portion of its surface. The coating includes a metallic layer which is modified by laser light, thereby marking the coating. This marking can be made in register with markings on the paper outside the coating, as shown in US-2008/0191462-A1. However, a problem with the method is that the marking is carried out on the surface of the document, wherefore the marked portions can easily be subjected to wear and degradation during use of the document, which can lead to doubts about its authenticity. Also, superficial markings can sometimes be subjected to fraudulent alterations.
The use of laser light for producing security features in security documents or elements is well known in the art.
For example, US-2010/0164217 teaches a method for manufacturing a security feature for a security element, a security paper or a data carrier that exhibits a substrate into which at least one through opening and at least one marking in register with the through opening are to be introduced.
US-2010/0272313-A1 teaches a forgery prevention medium that includes a volume hologram layer on which an interference pattern is recorded after being exposed to at least an emitted laser beam; a digital watermarking information layer on which digital watermarking information is recorded; and a substrate film.
WO-2009/106066-A1 discloses a security document including a layer with components sensitive to a laser light source, allowing for laser marking of the document.
JP-2005-279940-A discloses a printable security sheet, comprising a multilayer paper structure with an inner resin layer which can be altered by laser light.
US-2005/0142342-A1 discloses a process to increase the security level of paper documents. Applied to the paper document is a transfer film or laminating film having a laser-sensitive layer, and a laser-induced marking is produced in the laser-sensitive layer, for example, by laser-induced bleaching, laser-induced colour change or laser-induced blackening. It is taught that respective individualization of the document can be effected by way of that laser-induced marking.
US-2008/0187851-A1 discloses the marking of a material with identifier marks. An optical brightener is incorporated in the material, and marking is performed by reducing the brightness of the material at a selected location by directing local heating to this location, the mark thus produced appearing with a darker shade than its environment in ultraviolet light. The marking is based on partial or complete destruction of the brightening effect of the optical brightener under heating. The disclosure is suitable for providing e. g. coated paper and board containing an optical brightener with identifier marks for preventing falsifications.
WO-02/101147-A1 discloses a security thread with an opaque layer on which signs, figures or characters have been generated by means of a laser light, before the insertion of the security thread into a paper substrate.
EP-1291827-B1 and its U.S. counterpart US-2004/0232691-A1 disclose a method for the customization of security documents. They teach laser marking of at least two materials superimposed on a substrate, each of the at least two materials having different resistances to laser. An example is given in FIG. 3, where a first layer of material having a high resistance to laser and a second layer composed of two materials having different resistances are deposited on a banknote, and laser marked with a serial number of the banknote. The banknote also includes the serial number printed in another corner, and additional security elements such as a silver coated strip on which the serial number is provided, and a logo. According to the descriptions, the disclosure also makes it possible to provide a security thread or strip with a marking. The marking can be provided by the thread or strip manufacturer.
EP-2284015-B1 discloses a security element having a reflective layer which, by means of laser radiation, is marked with visually perceptible marks in the shape of patterns, letters, numbers or images.
EP-2271501-B1 teaches laser treatment of security documents involving perforation and simultaneous marking using a laser. A paper substrate has a marking region with a laser sensitive substance, and a security element is present in the marking region. The security element is weakened by laser light in order to generate, simultaneously and at perfect register, weak lines in the security element and marks in the paper substrate by alteration of the sensitive substance.
WO-2013/037473-A1 teaches marking of the front and/or rear surface of a substrate, or of the interior of a substrate, with laser light. The surface of a cylinder which is in contact with the substrate is arranged to take up ablated particles, so that these ablated particles do not adhere to the substrate or a following substrate.
US-2012/0103545-A1 relates to the production of colored markings on a substrate using laser light, and teaches combining a material for forming a colored mark under laser irradiation, and an antitampering agent. The substrate can be a paper substrate used in a security document and/or a security article.
U.S. Pat. No. 4,462,867-A relates to the production of paper for security documents. A security strip is used that has regions that obstruct drainage more than other regions. The strip can be provided with a pattern by cutting the pattern into the strip by, for example, a laser beam. This cutting takes place prior to the insertion of the strip into the paper. U.S. Pat. No. 4,462,867-A also seems to mention, as a possible material for the strip, some kind of paper. However, in view of how the strip is intended to be inserted, it is not clear how this would be possible in practice: the strip is introduced into the paper furnish, and no reference is made to any means or measures for preventing the strip from disintegrating in this rather hostile surrounding, before reaching the area where the paper is being formed. In this kind of paper making machines, the paper furnish generally includes mostly water, and is rather turbulent in order to make sure that the fibres of the paper will extend into different directions, thereby making the paper reasonably resistant to traction not only in the machine direction.
US-2006/0145468-A1 D4 refers to the production of a security document with at least one tangible marking in the form of a relief structure, produced by laser.